No (or low?) scart switch voltage

Background:
Running Gentoox Home 3.2 on a very early JPN box
(bought on release day).
Using Evox dash 3935 and an Evox M7 bios.
Installed to E via ftp
Running xfce, "composite" pal 800x600 hoc13, voc15.
The xbox is connected via Scart to a mechanical switch
box into a Sky+ receiver, then into a 4:3 JVC TV, using
RGB.
Xbox boot animation, Evox dash, and all games all set
the Scart pin 8 control signal so that the Sky box & TV
switch to display the Xbox automatically.

Problem:
As soon as I launch Gentoox from the dash the TV
switches back to showing analog TV. If I manually
select the AV0 channel on the TV then I can see the
Xbox output perfectly, in crisp RGB.

Hypothesis:
The Sky+ box must be seeing some voltage on pin8 and is
therefor switching to display the Xbox input, but my TV
doesn't see enough voltage to switch. (From previous
experience, I don't think my TV will switch to a 16:9
switch voltage (5v) but needs a 4:3 (12V) signal.)

Conclusion:
I think Gentoox may be configuring the video chip to
output a 16:9 switch voltage, when it should be setting
it to 4:3.

Hope this helps

Simon